Methods of controlling an electrical current to an inductive load such as a stepper motor are known. One such method for keeping a current at a constant level includes switching a power supply from an ON state (i.e., to supply power) to an OFF state (i.e., to cut the supply of power) based on a predetermined current threshold and a reference signal. By using such a method, the magnitude and cycle of the load current are kept constant, thereby enabling stable control of motor operation and reducing the possibility of the motor falling out of synchronization.
In harsh environments such as vehicle compartments, for example enclosed compartments or compartments in and around the engine bay where large temperature fluctuations may occur, the characteristics of the motor (i.e., load) may change based on temperature fluctuations. In such cases, a rise in temperature increases the resistance of the load and slows the rise and fall of the load current waveform, i.e. the wave formed by measured current values as a function of time. Likewise, a drop in temperature decreases the resistance of the load and quickens the rise and fall of the load current waveform. Changes in the load current cycle affect the timing of the threshold current detection which is based on the reference signal. As such, current detection timing based on the reference signal can be delayed such that the timing of the power supply switching (i.e. switching between ON and OFF states) can be delayed by one cycle. As a result, to stabilize motor control operation when current detection and load characteristics are affected by a rise in temperature, as described above, the output drive signal is twice as long as the reference drive signal. As such, the drive signal to signal the supply of power to a load affected by rising temperature levels is longer (e.g., twice as long) than the reference signal that determines the drive signal, which may cause a sub-harmonic oscillation of the drive signal when the reference signal ratio is above 50% and lowering the actual load current below a target value, thereby causing an unstable operation of the load.
In view of the above, an output voltage waveform is detected by a detector using motor phase detection (i.e., a phase detector) or an A/D converter, and a change of load characteristics due to temperature fluctuation is determined by detected waveform information (i.e., of the detected waveform), and the drive signal (i.e., a magnitude of the electric current and its frequency) is controlled accordingly. However, in such configuration, an appropriate feedback control may only be performable by using a microcomputer based on the A/D converter and the waveform information, as described in a patent document 1 listed below. Such configurations of the load driving apparatus are complex and expensive.
(Patent document 1) Japanese Patent Laid-Open No. 2004-350386